四(si)川盆地聚居(ju)著四(si)川省和重慶市大(da)部分人(ren)口(kou)、居(ju)民(min)主(zhu)要(yao)為漢族巴蜀民(min)系。是中(zhong)國和世界上人(ren)口(kou)最(zui)多(duo)的區域之一。
中國(guo)著名紅層盆地,中國(guo)各(ge)大盆地中形(xing)態最(zui)(zui)典型、緯度(du)最(zui)(zui)南、海拔(ba)最(zui)(zui)低的盆地。位于長江上游(you)地區,海拔(ba)500米左右,長江把(ba)它和東海一(yi)脈相(xiang)連(lian),它是中國(guo)最(zui)(zui)大的外流(liu)盆地。
四(si)川(chuan)盆(pen)地(di)(di)(di)(di)(di)的(de)(de)(de)面(mian)積約26萬(wan)余平(ping)(ping)方千米(mi)(面(mian)積是居第五第六位(wei)的(de)(de)(de)兩湖(hu)盆(pen)地(di)(di)(di)(di)(di)和(he)銀額(e)盆(pen)地(di)(di)(di)(di)(di)的(de)(de)(de)兩倍),占四(si)川(chuan)省面(mian)積的(de)(de)(de)46%。四(si)川(chuan)盆(pen)地(di)(di)(di)(di)(di)西依青藏高原(yuan)(yuan)和(he)橫(heng)斷山脈(mo),北近秦巴山脈(mo),與(yu)漢中盆(pen)地(di)(di)(di)(di)(di)相望,東(dong)接(jie)湘鄂西山地(di)(di)(di)(di)(di),南(nan)連云(yun)貴高原(yuan)(yuan),盆(pen)地(di)(di)(di)(di)(di)北緣(yuan)(yuan)(yuan)米(mi)倉山,南(nan)緣(yuan)(yuan)(yuan)大婁山,東(dong)緣(yuan)(yuan)(yuan)巫山,西緣(yuan)(yuan)(yuan)邛(qiong)崍山,西北邊緣(yuan)(yuan)(yuan)龍(long)門山,東(dong)北邊緣(yuan)(yuan)(yuan)大巴山,西南(nan)邊緣(yuan)(yuan)(yuan)大涼山,東(dong)南(nan)邊緣(yuan)(yuan)(yuan)相望于武陵(ling)山。這里(li)的(de)(de)(de)巖(yan)石,主要由紫(zi)(zi)(zi)紅色(se)砂(sha)巖(yan)和(he)頁巖(yan)組(zu)成(cheng)。這兩種巖(yan)石極易風化發育成(cheng)紫(zi)(zi)(zi)色(se)土(tu)。紫(zi)(zi)(zi)色(se)土(tu)含(han)有豐富的(de)(de)(de)鈣、磷、鉀等營養(yang)元素,是南(nan)方最(zui)(zui)肥(fei)沃的(de)(de)(de)自然土(tu)壤。四(si)川(chuan)盆(pen)地(di)(di)(di)(di)(di)是全國紫(zi)(zi)(zi)色(se)土(tu)分布最(zui)(zui)集中的(de)(de)(de)地(di)(di)(di)(di)(di)方,向有“紫(zi)(zi)(zi)色(se)盆(pen)地(di)(di)(di)(di)(di)”的(de)(de)(de)美稱(cheng)。四(si)川(chuan)盆(pen)地(di)(di)(di)(di)(di)底部面(mian)積約16萬(wan)多(duo)平(ping)(ping)方千米(mi),按(an)其(qi)地(di)(di)(di)(di)(di)理(li)差異,自西向東(dong)又可分為成(cheng)都平(ping)(ping)原(yuan)(yuan)、川(chuan)中丘陵(ling)和(he)川(chuan)東(dong)平(ping)(ping)行嶺谷(gu)三部分。成(cheng)都平(ping)(ping)原(yuan)(yuan)西方是龍(long)門山脈(mo),東(dong)方是龍(long)泉山脈(mo)。
四川(chuan)盆地可明(ming)顯分為(wei)邊緣山地和(he)盆地底部兩大部分,其面積分別(bie)約為(wei)10萬(wan)多和(he)16多萬(wan)平(ping)方公里。邊緣山地多中山和(he)低山。
景觀各要(yao)素過渡性明顯,如動植(zhi)(zhi)物(wu)(wu)組(zu)成(cheng)(cheng)上(shang)分別滲透了華(hua)中區(qu)、西南(nan)區(qu)、青(qing)藏(zang)高原(yuan)區(qu)和華(hua)北區(qu)的成(cheng)(cheng)分。邊緣山(shan)(shan)(shan)地(di)(di)區(qu)從(cong)下而上(shang)一般具有2~5個垂直(zhi)自然分帶。邊緣山(shan)(shan)(shan)地(di)(di)是(shi)四川多(duo)種經濟林木(mu)和用材林基地(di)(di)。農業上(shang)水土流(liu)失較為嚴(yan)重。盆地(di)(di)底部多(duo)丘陵、低(di)山(shan)(shan)(shan)和平原(yuan)。地(di)(di)表組(zu)成(cheng)(cheng)物(wu)(wu)質新而單一,多(duo)砂泥巖(yan)與第四紀沉(chen)積物(wu)(wu)。氣候上(shang)屬(shu)中亞熱(re)帶,熱(re)量遠比(bi)邊緣山(shan)(shan)(shan)地(di)(di)為高,但(dan)降(jiang)水量不及邊緣山(shan)(shan)(shan)地(di)(di)。植(zhi)(zhi)被為人工的次生林木(mu)所代替,覆被率低(di),耕地(di)(di)連片(pian),夏伏旱和洪(hong)澇是(shi)這一地(di)(di)區(qu)最大災害。
盆(pen)(pen)地(di)(di)內部地(di)(di)形地(di)(di)貌顯示出明顯的(de)三分特點(dian),即(ji)盆(pen)(pen)西平原(yuan)地(di)(di)貌、盆(pen)(pen)中丘陵地(di)(di)貌和盆(pen)(pen)東山地(di)(di)地(di)(di)貌,三者(zhe)以龍(long)泉(quan)山和華鎣山為界。
四川盆(pen)(pen)地(di)其(qi)輪廓形態總體近(jin)似(si)長(chang)方形,形似(si)信(xin)封狀,右傾斜置在亞洲大陸,因(yin)此被地(di)理(li)學(xue)者稱(cheng)之為信(xin)封盆(pen)(pen)地(di)。信(xin)封盆(pen)(pen)地(di)東部(bu)頂(ding)(ding)點(dian)云陽(靠近(jin)萬州(zhou)),南(nan)部(bu)頂(ding)(ding)點(dian)敘永(靠近(jin)遵義),西部(bu)頂(ding)(ding)點(dian)雅安(an),北部(bu)頂(ding)(ding)點(dian)廣元。信(xin)封盆(pen)(pen)地(di)幾何中(zhong)心城市遂寧(幾何中(zhong)心點(dian)大致在蓬溪南(nan)部(bu)地(di)區),中(zhong)北部(bu)(內頂(ding)(ding)角)中(zhong)心城市南(nan)充,中(zhong)南(nan)部(bu)內頂(ding)(ding)角安(an)岳。
四(si)川盆地西部(bu)的成都和東南部(bu)的重(zhong)慶皆為(wei)國家中(zhong)心城(cheng)市(shi)。
四(si)川盆(pen)地(di)由盆(pen)周山地(di)和盆(pen)地(di)底部構成。
盆(pen)周東部(bu)(bu)為(wei)(wei)長江三峽,南(nan)部(bu)(bu)為(wei)(wei)云貴高原(yuan),西部(bu)(bu)為(wei)(wei)青藏高原(yuan),北部(bu)(bu)為(wei)(wei)大巴(ba)山。
盆地底部主要分為川(chuan)東平(ping)行嶺谷、川(chuan)中丘陵和(he)川(chuan)西(xi)成都(dou)平(ping)原(yuan)三部分。龍泉(quan)山是成都(dou)平(ping)原(yuan)和(he)川(chuan)中丘陵的界山,華鎣山是川(chuan)中丘陵和(he)川(chuan)東平(ping)行嶺谷的界山。
(1)在(zai)龍(long)泉山(shan)和(he)龍(long)門山(shan)、邛崍山(shan)之間(jian)的盆西(xi)(xi)平(ping)(ping)(ping)(ping)原,系斷(duan)裂下陷由(you)岷(min)江(jiang)水系的河(he)流沖積而成(cheng),面積約8000平(ping)(ping)(ping)(ping)方(fang)千(qian)米(mi)(mi),為我國西(xi)(xi)南(nan)最(zui)大的平(ping)(ping)(ping)(ping)原,因成(cheng)都(dou)位于平(ping)(ping)(ping)(ping)原之中, 故(gu)稱(cheng)成(cheng)都(dou)平(ping)(ping)(ping)(ping)原。成(cheng)都(dou)平(ping)(ping)(ping)(ping)原海(hai)拔460米(mi)(mi)~750米(mi)(mi),地(di)勢(shi)由(you)西(xi)(xi)北向東(dong)南(nan)傾(qing)斜,地(di)表平(ping)(ping)(ping)(ping)坦(tan),相(xiang)對高差(cha)一(yi)般不超(chao)過30米(mi)(mi)~50米(mi)(mi),它由(you)岷(min)江(jiang)、沱江(jiang)、涪江(jiang)、青衣江(jiang)等八條河(he)流沖積聯(lian)綴而成(cheng),土壤肥沃,河(he)渠稠密(mi),有(you)著名的都(dou)江(jiang)堰自流灌溉,自古以來素有(you)“天府”之稱(cheng)。
(2)在龍泉(quan)山(shan)(shan)和華鎣山(shan)(shan)之間的(de)(de)盆(pen)中(zhong)丘(qiu)(qiu)陵,地勢低矮(ai),海拔大多(duo)在300米~500米之間,相對高(gao)差50米~150米,地勢由北(bei)向(xiang)南傾斜,巖層近于水平,在流水的(de)(de)長期侵蝕切割作(zuo)用下,形成(cheng)臺階(jie)狀的(de)(de)方山(shan)(shan)丘(qiu)(qiu)陵,南部(bu)(bu)多(duo)淺(qian)丘(qiu)(qiu),北(bei)部(bu)(bu)多(duo)深丘(qiu)(qiu)。丘(qiu)(qiu)陵地表軟硬(ying)相間的(de)(de)紫(zi)紅色(se)砂(sha)、頁巖極易風化為紫(zi)色(se)土,富含磷鉀,自然(ran)肥力較高(gao),宜種性廣,是四川全(quan)省糧食(shi)、經濟作(zuo)物(wu)主產區。
(3)華鎣(ying)山(shan)以東(dong)為(wei)盆東(dong)平行嶺谷區,由多條(tiao)近東(dong)北西南走(zou)向(xiang)的(de)條(tiao)狀背斜山(shan)地(di)與向(xiang)斜寬谷組成,山(shan)地(di)陡而窄,高700米(mi)(mi)-1000米(mi)(mi),其(qi)中,華鎣(ying)山(shan)高1705米(mi)(mi),為(wei)盆地(di)底部最(zui)高峰。山(shan)地(di)頂部的(de)石(shi)灰(hui)巖(yan)被雨水溶蝕后,常(chang)成凹(ao)槽,故山(shan)地(di)大(da)多具有“一山(shan)二嶺一槽”或“一山(shan)三嶺二槽”的(de)特(te)色。山(shan)嶺間的(de)谷地(di)寬而緩,海拔300米(mi)(mi)~500米(mi)(mi),其(qi)間丘陵、平壩交錯分布,是(shi)平行嶺谷區工農業生產主要(yao)地(di)區。
四川盆地邊緣山(shan)地區屬強烈(lie)上升的褶皺帶。
地(di)貌顯(xian)著特征是,海拔(ba)較高(gao)(gao),過(guo)渡性(xing)明顯(xian),均為一系(xi)列中山(shan)(shan)和(he)低(di)山(shan)(shan)所圍繞(rao)。盆地(di)北緣(yuan)米(mi)倉山(shan)(shan)、大巴山(shan)(shan)近東西走向(xiang),是秦巴山(shan)(shan)地(di)南翼部(bu)(bu)分(fen),海拔(ba)一般在(zai)(zai)1500米(mi)~2200米(mi)之間,山(shan)(shan)勢雄偉,山(shan)(shan)坡陡峭,溝谷(gu)深切,相對高(gao)(gao)差可(ke)達(da)500米(mi)~1000米(mi);南緣(yuan)大婁(lou)山(shan)(shan)屬氣勢磅(bang)礴的(de)云貴(gui)高(gao)(gao)原之一部(bu)(bu)分(fen);西緣(yuan)有(you)龍門山(shan)(shan)、邛崍山(shan)(shan)、峨(e)眉山(shan)(shan),山(shan)(shan)脊海拔(ba)都在(zai)(zai)1500米(mi)至~3000米(mi)以上,相對高(gao)(gao)差可(ke)達(da)1000米(mi),屬中國地(di)勢第一級(ji)階(jie)梯(ti)。峨(e)眉山(shan)(shan)頂峰高(gao)(gao)3099米(mi),與附近的(de)平原相對高(gao)(gao)差達(da)2660米(mi),山(shan)(shan)勢巍峨(e)秀麗(li),為中國四(si)大佛教名(ming)山(shan)(shan)之一。
四周為(wei)海拔(ba)2000~3000米(mi)的山(shan)(shan)脈和(he)高(gao)(gao)原(yuan)所環繞,北(bei)面(mian)(mian)是大(da)巴山(shan)(shan)、米(mi)倉山(shan)(shan)、龍門山(shan)(shan),西(xi)面(mian)(mian)是青藏高(gao)(gao)原(yuan)邊緣的邛崍山(shan)(shan)、大(da)涼(liang)山(shan)(shan),南(nan)面(mian)(mian)是大(da)婁山(shan)(shan),東面(mian)(mian)是巫山(shan)(shan)。
無論從(cong)構(gou)造還是(shi)(shi)人地(di)(di)(di)(di)(di)貌上(shang)看,四(si)川(chuan)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)都是(shi)(shi)一個典型的(de)(de)(de)(de)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)。它(ta)從(cong)震旦紀以(yi)來就是(shi)(shi)地(di)(di)(di)(di)(di)殼比較穩定的(de)(de)(de)(de)大(da)(da)型拗陷(xian)區(qu)。晚(wan)三迭(die)紀的(de)(de)(de)(de)印支運動中(zhong)(zhong)成(cheng)(cheng)為(wei)一個內(nei)陸湖盆(pen)(pen)(pen),但范圍要比今(jin)日(ri)的(de)(de)(de)(de)四(si)川(chuan)大(da)(da)得多(duo),中(zhong)(zhong)生(sheng)代(dai)堆積(ji)(ji)了(le)厚達3000~4000米的(de)(de)(de)(de)紫紅色的(de)(de)(de)(de)砂(sha)巖和(he)頁巖,因此人們又(you)稱它(ta)為(wei)“紅色盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)”或“紫色盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)”。中(zhong)(zhong)生(sheng)代(dai)末期的(de)(de)(de)(de)四(si)川(chuan)運動使(shi)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)周(zhou)圍褶皺(zhou)成(cheng)(cheng)山(shan),中(zhong)(zhong)間相(xiang)對(dui)下(xia)陷(xian),輪(lun)廓基本形(xing)(xing)成(cheng)(cheng),盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)內(nei)部(bu)地(di)(di)(di)(di)(di)層也發生(sheng)大(da)(da)規模的(de)(de)(de)(de)變形(xing)(xing)。東部(bu)出現(xian)(xian)一組北(bei)(bei)東向(xiang)的(de)(de)(de)(de)褶皺(zhou),稱盆(pen)(pen)(pen)東褶皺(zhou)帶(dai)(dai);中(zhong)(zhong)部(bu)形(xing)(xing)成(cheng)(cheng)穹窿(long)構(gou)造,稱盆(pen)(pen)(pen)中(zhong)(zhong)穹窿(long)帶(dai)(dai);西(xi)部(bu)表(biao)現(xian)(xian)成(cheng)(cheng)為(wei)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)沉陷(xian)帶(dai)(dai),為(wei)今(jin)天盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)的(de)(de)(de)(de)三個地(di)(di)(di)(di)(di)貌區(qu)奠定了(le)基礎。新生(sheng)代(dai)的(de)(de)(de)(de)喜馬拉雅(ya)運動使(shi)周(zhou)圍山(shan)地(di)(di)(di)(di)(di)再次(ci)上(shang)升(sheng),盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)再次(ci)相(xiang)對(dui)下(xia)陷(xian), 成(cheng)(cheng)都一帶(dai)(dai)下(xia)陷(xian)更深(shen)。同時(shi),長(chang)江(jiang)切穿巫山(shan),滾滾東流(liu),完成(cheng)(cheng)了(le)統一的(de)(de)(de)(de)長(chang)江(jiang)水系,使(shi)內(nei)流(liu)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)轉變為(wei)外(wai)流(liu)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)。四(si)川(chuan)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)可明顯分為(wei)邊緣山(shan)地(di)(di)(di)(di)(di)和(he)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)底部(bu)兩大(da)(da)部(bu)分,其面積(ji)(ji)分別約(yue)為(wei)10萬(wan)多(duo)和(he)16萬(wan)多(duo)平方千米。 四(si)川(chuan)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)是(shi)(shi)我國最大(da)(da)的(de)(de)(de)(de)外(wai)流(liu)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di),面積(ji)(ji)26萬(wan)多(duo)平方千米。四(si)川(chuan)盆(pen)(pen)(pen)地(di)(di)(di)(di)(di)內(nei)部(bu)丘陵、平原交錯,地(di)(di)(di)(di)(di)勢北(bei)(bei)高南低。由(you)于地(di)(di)(di)(di)(di)表(biao)形(xing)(xing)態的(de)(de)(de)(de)不同,以(yi)華鎣山(shan)、龍(long)泉山(shan)為(wei)界(jie)。
盆(pen)底(di)可分為三部分:
①華鎣山以東為大致平(ping)行的(de)川東嶺谷,由(you)東北—西南走向(xiang)的(de)許多條狀山體組成,海拔一般在(zai)700~800米(mi)(mi),谷地(di)中多低丘與平(ping)壩,海拔200~500米(mi)(mi),是川東農業和人(ren)口集中的(de)地(di)方(fang)。
②華鎣山(shan)(shan)(shan)和龍泉山(shan)(shan)(shan)之間為(wei)方(fang)山(shan)(shan)(shan)丘(qiu)陵(ling),即川中丘(qiu)陵(ling)。區內(nei)由于紫紅色砂頁巖傾角(jiao)平(ping)緩,受切割(ge)后(hou)形成(cheng)大片方(fang)山(shan)(shan)(shan)式(shi)丘(qiu)陵(ling)。海拔350~450米(mi),相對高(gao)度幾十(shi)米(mi)。當(dang)地勞動人民利用方(fang)山(shan)(shan)(shan)山(shan)(shan)(shan)丘(qiu)土(tu)層深厚的特點,把梯田一(yi)直修到(dao)山(shan)(shan)(shan)頂。
③龍(long)泉山以西(xi)(xi)為平(ping)原(yuan)。稱為川西(xi)(xi)平(ping)原(yuan)或(huo)成都平(ping)原(yuan),面積6000多(duo)平(ping)方千米,是四川盆地最大(da)的平(ping)原(yuan),也是西(xi)(xi)南地區(qu)最大(da)的平(ping)原(yuan),海拔約600米。
四(si)(si)(si)川(chuan)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)屬揚(yang)子準地(di)(di)(di)(di)(di)(di)(di)臺四(si)(si)(si)川(chuan)臺坳(ao)。古(gu)生代(dai)時(shi)相(xiang)對(dui)隆(long)起,缺乏泥盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)系和(he)石(shi)(shi)炭系。印支(zhi)運(yun)(yun)動(dong)轉換為大(da)型(xing)拗陷,晚燕山(shan)(shan)運(yun)(yun)動(dong),特(te)別是(shi)喜(xi)馬拉雅(ya)(ya)運(yun)(yun)動(dong)后發(fa)生褶(zhe)皺隆(long)起。盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)的(de)格局主要受(shou)北(bei)東(dong)—南(nan)西(xi)(xi)向及北(bei)西(xi)(xi)向兩條構造線(xian)控制,構成(cheng)(cheng)(cheng)了(le)典型(xing)的(de)菱(ling)(ling)形(xing)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di),四(si)(si)(si)川(chuan)廣(guang)元、四(si)(si)(si)川(chuan)雅(ya)(ya)安、四(si)(si)(si)川(chuan)敘永為菱(ling)(ling)形(xing)的(de)四(si)(si)(si)頂點,東(dong)西(xi)(xi)兩邊稍(shao)長,為380~430公(gong)里(li),南(nan)北(bei)兩邊略短,為310~330公(gong)里(li)。以(yi)上(shang)菱(ling)(ling)形(xing)四(si)(si)(si)頂點的(de)連(lian)線(xian)與(yu)(yu)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)內(nei)650~750米(mi)的(de)等(deng)高(gao)線(xian)大(da)體(ti)相(xiang)當,盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)底(di)部(bu)(bu)(bu)與(yu)(yu)邊緣山(shan)(shan)地(di)(di)(di)(di)(di)(di)(di)也以(yi)此(ci)為分界。四(si)(si)(si)川(chuan)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)在(zai)距今(jin)(jin)1.4億年以(yi)前(qian),還是(shi)內(nei)陸(lu)湖(hu)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)。至距今(jin)(jin)6.6千萬年時(shi),盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)邊緣山(shan)(shan)地(di)(di)(di)(di)(di)(di)(di)迅速隆(long)升,長江中(zhong)上(shang)游水系開始溝通。盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)內(nei)湖(hu)水東(dong)瀉奠定(ding)了(le)現(xian)今(jin)(jin)之(zhi)(zhi)地(di)(di)(di)(di)(di)(di)(di)貌(mao)形(xing)態(tai)。盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)邊緣多(duo)低山(shan)(shan)和(he)中(zhong)山(shan)(shan),山(shan)(shan)勢陡峻,發(fa)源盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)邊緣山(shan)(shan)地(di)(di)(di)(di)(di)(di)(di)的(de)河流(liu)(liu)大(da)多(duo)為“V”型(xing)谷,嶺谷高(gao)差都逾500~1 000米(mi), 地(di)(di)(di)(di)(di)(di)(di)表崎嶇,故歷史(shi)上(shang)就(jiu)有(you)“蜀道難(nan),難(nan)于上(shang)青天(tian)”之(zhi)(zhi)說。山(shan)(shan)脊海(hai)拔(ba)大(da)多(duo)在(zai)2000~3000米(mi),西(xi)(xi)北(bei)部(bu)(bu)(bu)與(yu)(yu)西(xi)(xi)部(bu)(bu)(bu)可超過3000~4000米(mi),如龍門山(shan)(shan)4984米(mi),峨眉山(shan)(shan)3099米(mi),小相(xiang)嶺4 791米(mi)。地(di)(di)(di)(di)(di)(di)(di)表廣(guang)泛出露古(gu)生代(dai)及其以(yi)前(qian)的(de)石(shi)(shi)灰巖(yan)(yan)(yan),其次(ci)為板巖(yan)(yan)(yan)、片巖(yan)(yan)(yan)、結晶灰巖(yan)(yan)(yan)、石(shi)(shi)英巖(yan)(yan)(yan)、砂泥巖(yan)(yan)(yan)和(he)礫巖(yan)(yan)(yan),局部(bu)(bu)(bu)有(you)花崗巖(yan)(yan)(yan)和(he)玄武巖(yan)(yan)(yan)。石(shi)(shi)灰巖(yan)(yan)(yan)分布(bu)區可見石(shi)(shi)林、溶(rong)洞(dong)、暗河、槽(cao)谷等(deng)喀斯特(te)地(di)(di)(di)(di)(di)(di)(di)貌(mao),盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)南(nan)緣興(xing)文縣素有(you)“石(shi)(shi)林洞(dong)鄉”之(zhi)(zhi)稱(見興(xing)文石(shi)(shi)林)。巫山(shan)(shan)十二峰(feng)和(he)金佛山(shan)(shan)等(deng)名山(shan)(shan)主要也由石(shi)(shi)灰巖(yan)(yan)(yan)發(fa)育而(er)成(cheng)(cheng)(cheng)。由石(shi)(shi)灰巖(yan)(yan)(yan)、玄武巖(yan)(yan)(yan)、花崗巖(yan)(yan)(yan)等(deng)組(zu)成(cheng)(cheng)(cheng)的(de)峨眉山(shan)(shan)及由砂泥巖(yan)(yan)(yan)、礫巖(yan)(yan)(yan)組(zu)成(cheng)(cheng)(cheng)的(de)青城山(shan)(shan),素有(you)“峨眉天(tian)下秀”、“青城天(tian)下幽”之(zhi)(zhi)稱,為中(zhong)國著名游覽勝地(di)(di)(di)(di)(di)(di)(di)。盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)底(di)部(bu)(bu)(bu)海(hai)拔(ba)多(duo)數(shu)在(zai)250~700米(mi),地(di)(di)(di)(di)(di)(di)(di)勢東(dong)南(nan)傾,盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)內(nei)各(ge)河流(liu)(liu)均由邊緣山(shan)(shan)地(di)(di)(di)(di)(di)(di)(di)匯聚(ju)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)底(di)部(bu)(bu)(bu)的(de)長江干流(liu)(liu),形(xing)成(cheng)(cheng)(cheng)向心狀水系。地(di)(di)(di)(di)(di)(di)(di)表為大(da)面積(ji)的(de)中(zhong)生代(dai)紫紅色砂巖(yan)(yan)(yan)與(yu)(yu)泥巖(yan)(yan)(yan)所(suo)覆蓋,故稱為“紅層盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)”,是(shi)中(zhong)國中(zhong)生代(dai)陸(lu)相(xiang)紅層分布(bu)最集(ji)中(zhong)地(di)(di)(di)(di)(di)(di)(di)區。四(si)(si)(si)川(chuan)盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di)為丘(qiu)陵(ling)性盆(pen)(pen)(pen)(pen)(pen)(pen)(pen)地(di)(di)(di)(di)(di)(di)(di),底(di)部(bu)(bu)(bu)以(yi)丘(qiu)陵(ling)為主,次(ci)為低山(shan)(shan)和(he)平原。
四(si)川盆(pen)地(di)屬揚子陸臺一部分,稱為(wei)(wei)四(si)川陸臺,屬較穩定的(de)地(di)區,但仍經(jing)過兩次(ci)大規模(mo)的(de)海(hai)浸。第(di)一次(ci)從5億多年(nian)前的(de)寒(han)武(wu)紀開始,延續到(dao)3.7億多年(nian)的(de)志留紀,不(bu)斷下陷成(cheng)(cheng)了海(hai)洋盆(pen)地(di),志留紀時(shi)發(fa)生(sheng)加里東運動,除了西部的(de)龍門山地(di)槽繼續下陷外(wai),其余(yu)地(di)區上升為(wei)(wei)陸。2.7億年(nian)前的(de)石炭(tan)紀末,發(fa)生(sheng)范圍更大的(de)第(di)二(er)次(ci)海(hai)浸,盆(pen)地(di)再次(ci)為(wei)(wei)海(hai)洋占據。二(er)疊(die)紀時(shi)海(hai)陸交替,形成(cheng)(cheng)重慶(qing)附近的(de)南酮、松藻(zao)、天府等煤礦。二(er)疊(die)紀末,盆(pen)地(di)西部巖漿噴出,峨眉山小(xiao)金頂及(ji)清音(yin)閣一帶的(de)玄武(wu)巖就在這時(shi)生(sheng)成(cheng)(cheng)。
距今(jin)1.9億(yi)(yi)年的三疊紀,“印支運動”使盆地邊緣逐漸隆起(qi)成(cheng)山(shan),被海(hai)(hai)水(shui)(shui)淹沒的地區逐漸上升成(cheng)陸,由海(hai)(hai)盆轉為湖(hu)盆。當(dang)時(shi)湖(hu)水(shui)(shui)幾乎占(zhan)據(ju)現今(jin)四川盆地的全境,稱(cheng)為“蜀湖(hu)”,從此結(jie)束(shu)了(le)海(hai)(hai)浸的歷史。在中生代(dai)漫(man)長(chang)的1億(yi)(yi)多年里,盆地氣候(hou)溫(wen)暖濕(shi)潤,到(dao)處生長(chang)蕨類、蘇鐵和(he)(he)裸子植物,是(shi)又(you)一(yi)(yi)(yi)個(ge)成(cheng)煤期(qi),永(yong)榮煤礦即在三疊紀和(he)(he)侏(zhu)羅(luo)紀時(shi)形成(cheng)。東(dong)起(qi)長(chang)壽、墊江,西(xi)到(dao)江油、邛(qiong)崍,北抵大巴山(shan)麓,南到(dao)貴(gui)州赤水(shui)(shui),還是(shi)天然(ran)氣富集區。這一(yi)(yi)(yi)時(shi)期(qi)爬行動物恐(kong)龍稱(cheng)霸一(yi)(yi)(yi)時(shi)。1957年在合(he)(he)州發(fa)現的“合(he)(he)州馬門溪龍”身長(chang)22米(mi),高3.5米(mi),是(shi)中國亞洲最(zui)大和(he)(he)最(zui)完(wan)整的恐(kong)龍化石。
7000萬(wan)年(nian)前(qian)(qian)(qian)的(de)白堊紀末期(qi),發生(sheng)又一次強(qiang)烈(lie)的(de)地(di)殼運(yun)(yun)動(dong)“燕山(shan)運(yun)(yun)動(dong)”。盆(pen)(pen)地(di)四(si)周山(shan)地(di)繼續隆起,同(tong)時產生(sheng)不少大(da)(da)(da)斷層(ceng),如西(xi)部的(de)龍門山(shan)大(da)(da)(da)斷層(ceng)和東部的(de)華瑩山(shan)大(da)(da)(da)斷層(ceng),把(ba)盆(pen)(pen)地(di)分為(wei)三部分。蜀(shu)湖(hu)縮小為(wei)僅有2萬(wan)平(ping)方(fang)公里(li)的(de)蜀(shu)湖(hu)。封閉(bi)的(de)盆(pen)(pen)地(di)地(di)形及急劇縮小的(de)水面,使氣候逐(zhu)漸變(bian)得干熱,沉(chen)積物由(you)海相(xiang)、海陸交替(ti)相(xiang)變(bian)為(wei)陸相(xiang),大(da)(da)(da)量(liang)風(feng)化、侵(qin)蝕(shi)、剝(bo)蝕(shi)的(de)物質在盆(pen)(pen)地(di)堆積了數(shu)千米(mi)厚,形成(cheng)(cheng)紅色和紫紅色的(de)砂、泥、頁巖。裸(luo)子植物不斷衰(shuai)退,恐龍滅(mie)絕了。內陸湖(hu)泊在干燥條件下,經(jing)強(qiang)烈(lie)蒸發,濃度增大(da)(da)(da),鹽(yan)分不斷積累,形成(cheng)(cheng)鹽(yan)湖(hu),后來泥沙掩埋而保存于地(di)層(ceng)之(zhi)中(zhong),經(jing)過漫(man)長(chang)的(de)地(di)質作(zuo)用(yong)形成(cheng)(cheng)巖層(ceng),自(zi)貢一帶是著(zhu)名的(de)井鹽(yan)產地(di)。 2000多萬(wan)年(nian)前(qian)(qian)(qian)的(de)新第(di)三紀,受喜(xi)馬拉雅造(zao)山(shan)運(yun)(yun)動(dong)的(de)影響(xiang)。距今(jin)二(er)、三百萬(wan)年(nian)的(de)第(di)四(si)紀,地(di)殼再次發生(sheng)構造(zao)運(yun)(yun)動(dong)。從(cong)而,四(si)川(chuan)(chuan)盆(pen)(pen)地(di)由(you)內流(liu)(liu)盆(pen)(pen)地(di)變(bian)為(wei)外流(liu)(liu)陸盆(pen)(pen),由(you)封閉(bi)的(de)內流(liu)(liu)區(qu)變(bian)為(wei)外流(liu)(liu)區(qu),由(you)以堆積為(wei)主變(bian)為(wei)侵(qin)蝕(shi)為(wei)主,經(jing)歷了海盆(pen)(pen)——湖(hu)盆(pen)(pen)——陸盆(pen)(pen)的(de)滄桑之(zhi)變(bian)。 第(di)四(si)紀是冰川(chuan)(chuan)廣布的(de)時代,盆(pen)(pen)地(di)西(xi)北山(shan)地(di)發育大(da)(da)(da)量(liang)冰川(chuan)(chuan)。冰川(chuan)(chuan)消(xiao)融后,大(da)(da)(da)量(liang)沉(chen)積物由(you)岷(min)江(jiang)、沱江(jiang)等攜帶,堆積在西(xi)部的(de)凹陷區(qu),即以前(qian)(qian)(qian)的(de)蜀(shu)湖(hu)之(zhi)中(zhong),最終(zhong)形成(cheng)(cheng)了成(cheng)(cheng)都(dou)平(ping)原。 四(si)川(chuan)(chuan)盆(pen)(pen)地(di)地(di)貌(mao)豐富,平(ping)原7%,丘陵(ling)52%,低山(shan)41%。
四(si)川盆(pen)地(di)中植(zhi)物(wu)近萬種,古老而特(te)有(you)(you)種之多(duo)為中國其(qi)他地(di)區所不(bu)及。在盆(pen)地(di)邊(bian)緣山(shan)地(di)及盆(pen)東平行(xing)嶺谷尚可見水杉、銀杉、鵝掌楸、檫木(mu)、三尖杉、珙桐、水青樹(shu)、連香樹(shu)、領春木(mu)、金錢(qian)槭、蠟梅、杜仲、紅(hong)豆杉、鐘萼木(mu)、福建(jian)柏、穗花(hua)杉、崖柏、木(mu)瓜(gua)紅(hong)等(deng)珍稀(xi)孑遺植(zhi)物(wu)與特(te)有(you)(you)種。在濕熱(re)河谷可見桫(suo)欏、小羽桫(suo)欏、烏毛蕨、華南紫(zi)萁、里(li)白等(deng)古熱(re)帶(dai)(dai)孑遺植(zhi)物(wu)。 四(si)川盆(pen)地(di)的地(di)帶(dai)(dai)性植(zhi)被是亞熱(re)帶(dai)(dai)常(chang)綠(lv)闊葉(xie)(xie)(xie)林(lin)(lin),其(qi)代(dai)表(biao)樹(shu)種有(you)(you)栲(kao)樹(shu)、峨眉栲(kao)、刺(ci)果(guo)米櫧、青岡、曼(man)青岡、包石櫟、華木(mu)荷(he)(he)、大包木(mu)荷(he)(he)、四(si)川大頭茶、楨楠、潤楠等(deng),海拔一般(ban)情況下在1600~1 800米以下。其(qi)次有(you)(you)馬(ma)尾松、杉木(mu)、柏木(mu)組成(cheng) 的亞熱(re)帶(dai)(dai)針(zhen)葉(xie)(xie)(xie)林(lin)(lin)及竹林(lin)(lin)。 邊(bian)緣山(shan)地(di)從下而上是常(chang)綠(lv)闊葉(xie)(xie)(xie)林(lin)(lin)、常(chang)綠(lv)闊葉(xie)(xie)(xie)與落(luo)葉(xie)(xie)(xie)闊葉(xie)(xie)(xie)混交林(lin)(lin),寒溫帶(dai)(dai)山(shan)地(di)針(zhen)葉(xie)(xie)(xie)林(lin)(lin),局部有(you)(you)亞高山(shan)灌叢草甸。
四(si)川(chuan)盆地(di)是(shi)中(zhong)國(guo)動(dong)(dong)物(wu)種類(lei)最(zui)多、最(zui)齊全(quan)的(de)地(di)區(qu)之一。據(ju)統計(ji),除(chu)魚類(lei)外,盆地(di)底部共有動(dong)(dong)物(wu)417種,盆地(di)西(xi)(xi)緣、北(bei)緣和南緣山地(di)分(fen)別(bie)為487種、317種與288種,其中(zhong)經濟動(dong)(dong)物(wu)均(jun)占(zhan)一半以上。盆地(di)西(xi)(xi)緣山地(di)是(shi)中(zhong)國(guo)特有而古老動(dong)(dong)物(wu)保存最(zui)好、最(zui)集中(zhong)的(de)地(di)區(qu),屬于一類(lei)保護動(dong)(dong)物(wu)的(de)有大熊貓(mao)、金(jin)(jin)絲猴(hou)、扭(niu)角羚、灰金(jin)(jin)絲猴(hou)、白(bai)唇鹿等(deng)。還有珍貴(gui)特有動(dong)(dong)物(wu)小熊貓(mao)、雪(xue)豹、鬣羚、短尾猴(hou)、獼猴(hou)、毛冠鹿、水獺及(ji)鴛鴦、血雉(zhi)、紅(hong)腹(fu)角雉(zhi)、綠尾虹雉(zhi)、白(bai)腹(fu)錦雞(ji)、紅(hong)腹(fu)錦雞(ji)等(deng)。 盆地(di)西(xi)(xi)緣的(de)平(ping)武、青(qing)川(chuan)、北(bei)川(chuan)、寶(bao)興(xing)、天全(quan)、洪雅、馬邊(bian)等(deng)地(di),均(jun)為中(zhong)國(guo)大熊貓(mao)的(de)主要分(fen)布區(qu)。已(yi)設立唐家河(he)、王朗(lang)、臥(wo)龍(見臥(wo)龍自然保護區(qu))、蜂(feng)桶(tong)寨(zhai)、喇叭(ba)河(he)等(deng)7處(chu)大熊貓(mao)、金(jin)(jin)絲猴(hou)等(deng)自然保護區(qu)。 酉(you)陽、馬邊(bian)、平(ping)武等(deng)盆地(di)邊(bian)緣山地(di)溪溝(gou)中(zhong)的(de)大鯢及(ji)長江、金(jin)(jin)沙江中(zhong)的(de)中(zhong)華鱘(xun)、白(bai)鱘(xun)也為四(si)川(chuan)所特有,均(jun)屬國(guo)家保護動(dong)(dong)物(wu)。
四川盆地有煤、鐵、天然(ran)氣(qi)、石油、鹽、芒(mang)硝、石膏(gao)、磷、硫、鋁、金、銅(tong)、錳、石墨、汞等(deng)礦產(chan)資源,其中天然(ran)氣(qi)、芒(mang)硝為全(quan)中國(guo)之冠,并有中國(guo)重要的(de)(de)鍶(si)礦。川東北是世界最大的(de)(de)天然(ran)氣(qi)富集區之一,川中和南充(chong)等(deng)地鹽巖(yan)礦儲量(liang)很(hen)大。
四川盆地(di)地(di)形(xing)閉塞,由于(yu)(yu)北(bei)部秦嶺阻擋(dang)冷空氣(qi),冬季氣(qi)溫(wen)高(gao)于(yu)(yu)同緯(wei)(wei)度(du)(du)(du)其他地(di)區。最(zui)(zui)冷月(yue)北(bei)部均溫(wen)3~5℃,南部5~8℃,較(jiao)同緯(wei)(wei)度(du)(du)(du)的(de)上海、湖北(bei)、安徽及緯(wei)(wei)度(du)(du)(du)偏南的(de)貴州高(gao)1~4℃。盆地(di)北(bei)部極(ji)端最(zui)(zui)低溫(wen)-8~-5℃,南部-5℃~-2℃。霜(shuang)雪少(shao)見,年(nian)無霜(shuang)期長280~350天,同緯(wei)(wei)度(du)(du)(du)上的(de)武漢(han)冬天卻霜(shuang)雪不斷,盆地(di)位于(yu)(yu)長江河谷中的(de)長寧全年(nian)無霜(shuang)。 盆地(di)各地(di)夏季始于(yu)(yu)5月(yue)底,終于(yu)(yu)9月(yue)中旬(xun),夏長4個月(yue),溫(wen)度(du)(du)(du)上東高(gao)西(xi)低。盆地(di)西(xi)部夏天平(ping)均氣(qi)溫(wen)約(yue)25℃,較(jiao)同緯(wei)(wei)度(du)(du)(du)的(de)上海、武漢(han)等城市偏低3-4度(du)(du)(du),但(dan)濕度(du)(du)(du)較(jiao)大(da),所以悶熱難忍。
東部最(zui)熱(re)(re)月(yue)氣(qi)溫(wen)(wen)高(gao)(gao)(gao)(gao)達26~29℃,長(chang)江(jiang)河(he)谷(gu)局地達到30℃。盛(sheng)夏連晴(qing)高(gao)(gao)(gao)(gao)溫(wen)(wen)天氣(qi)又造成盆地東南(nan)(nan)(nan)部嚴重的(de)夏伏旱(han)。盆地氣(qi)溫(wen)(wen)東南(nan)(nan)(nan)高(gao)(gao)(gao)(gao)西北低,盆底高(gao)(gao)(gao)(gao)邊緣(yuan)(yuan)低; 各地年均溫(wen)(wen)16~18℃。10℃以上活動積溫(wen)(wen)4500~6000℃,持續期8~9個月(yue),屬中(zhong)亞(ya)熱(re)(re)帶(dai)。東南(nan)(nan)(nan)部的(de)長(chang)江(jiang)河(he)谷(gu)積溫(wen)(wen)超過6000℃,相(xiang)當于中(zhong)國(guo)南(nan)(nan)(nan)嶺以南(nan)(nan)(nan)的(de)南(nan)(nan)(nan)亞(ya)熱(re)(re)帶(dai)氣(qi)候。盆地氣(qi)溫(wen)(wen)東高(gao)(gao)(gao)(gao)西低,南(nan)(nan)(nan)高(gao)(gao)(gao)(gao)北低,盆底高(gao)(gao)(gao)(gao)而邊緣(yuan)(yuan)低,等溫(wen)(wen)線(xian)分布(bu)呈現同心圓狀。盆地邊緣(yuan)(yuan)山(shan)地氣(qi)溫(wen)(wen)具有垂(chui)直分布(bu)特點,如峨(e)眉山(shan)、金佛(fo)山(shan)海拔升高(gao)(gao)(gao)(gao)百米(mi),氣(qi)溫(wen)(wen)遞減0.55℃和0.61℃。峨(e)眉山(shan)頂年均溫(wen)(wen)僅3℃,10℃以上活動積溫(wen)(wen)586℃,氣(qi)候上相(xiang)當于寒(han)溫(wen)(wen)帶(dai)和亞(ya)寒(han)帶(dai)。
四(si)川盆(pen)(pen)地(di)(di)(di)年(nian)降(jiang)水量(liang)(liang)(liang)1000~1300毫米,盆(pen)(pen)地(di)(di)(di)邊緣山(shan)(shan)地(di)(di)(di)降(jiang)水十(shi)分充沛,如樂山(shan)(shan)和雅安(an)間的(de)(de)西緣山(shan)(shan)地(di)(di)(di)年(nian)降(jiang)水量(liang)(liang)(liang)為(wei)(wei)1 500~1800毫米,為(wei)(wei)中國(guo)突(tu)出的(de)(de)多雨(yu)區(qu)(qu),有“華西雨(yu)屏(ping)”之(zhi)稱。但冬干、春旱、夏澇、秋綿(mian)雨(yu),年(nian)內分配不(bu)均(jun),70~75%的(de)(de)雨(yu)量(liang)(liang)(liang)集中于6~10月。最大日降(jiang)水量(liang)(liang)(liang)可達(da)300~500毫米。“巴(ba)山(shan)(shan)夜雨(yu)”自古聞名,夜雨(yu)占總雨(yu)量(liang)(liang)(liang)的(de)(de)60~70%以上。盆(pen)(pen)地(di)(di)(di)區(qu)(qu)霧大濕(shi)重,云(yun)低陰天(tian)多。峨眉山(shan)(shan)、金佛(fo)山(shan)(shan)是中國(guo)霧日最多地(di)(di)(di)區(qu)(qu)之(zhi)一(yi),年(nian)相對濕(shi)度之(zhi)高(gao)也為(wei)(wei)中國(guo)之(zhi)冠。盆(pen)(pen)地(di)(di)(di)年(nian)日照僅900~1300小時,年(nian)太陽輻射量(liang)(liang)(liang)為(wei)(wei)370~420千焦耳/平方(fang)厘(li)米,均(jun)為(wei)(wei)中國(guo)最低值(zhi),故有“蜀犬(quan)吠(fei)日”之(zhi)說。